CHAPTER 7. Conclusions and Recommendations

Conclusions

NIRAS measurements have shown a clear distinction between reactive and nonreactive aggregates and among concrete mixtures containing reactive aggregate with and without mitigation for ASR. For HR cases, there is evidence of earlier detection of ASR-related damage with the NIRAS testing technique than with concrete prism expansion.

The NIRAS measurement setup has proven to be a robust and accurate measurement technique. The variability between successive measurements has been shown to be less than 20 percent overall and less than 10 percent with a prepared adhesion surface.

NIRAS has also been proven to be a powerful nondestructive testing (NDT) tool to rapidly detect microcrack-type damage (regardless of the cause) in concrete in an early stage of the material degradation. Based on the current body of research, this test should be used as a supplement to other standard testing. Additional research and testing can be used to assess whether NIRAS can be used as an alternative test.

With some aggregate types, a higher rate of expansion is measured with aggregate that has been crushed than in the as-received condition, showing a potential effect of aggregate size. The NIRAS measurements, however, do not appear to be affected by the reactive aggregate size and show the potential to be used to evaluate job-specific aggregates, while also eliminating the need for the time-consuming crushing and grading processes.

Complementary petrography, performed by staining the concrete samples with uranyl acetate, was used to confirm the presence of ASR gel and cracking within the samples. Based on this lack of fluorescence, gel, and cracking, one aggregate identified as non-reactive through NIRAS was confirmed to be non-reactive, despite having been identified as potentially deleterious and reactive through expansion testing. Further petrographic analysis to complement expansion and NIRAS testing will contribute to improving the understanding of ASR as well as to improving the understanding of the correlation between the results of NIRAS and expansion testing.

Two nonlinearity parameters are defined: one (η) that characterizes the rate of damage development at a certain age and another (ηc) that characterizes the amount of damage accumulated up until that time. Based on the experimental results for a limited number of aggregates—albeit with various levels of reactivity—it is proposed that the aggregate under assessment can be considered alkali reactive if the nonlinearity parameter of its concrete prism specimen measures 0.2 or more at any time during the 12-month test period. Nonlinearity parameters between 0.05 and 0.2 may suggest some potential for ASR, and the aggregate should be further evaluated. These proposed limits, however, must be verified through a more comprehensive evaluation of a broader range of aggregates and also through round-robin testing.

Recommendations for Further Research

This research has shown that the NIRAS technique has potential as a powerful method for non-destructive evaluation of concrete in the laboratory, in cored samples, and potentially in the field. Although applications for detection of ASR-induced damage are the focus of this research, results here suggest the technique could be further developed for detection and quantification of cumulative damage due to various causes (e.g., fatigue, freeze/thaw, sulfate attack, reinforcement corrosion) and also for examination of hydration and self-healing in concrete.

With regard to ASR, further efforts are needed in several areas. First, additional research is needed for continued development and validation of the NIRAS technique with a broader range of aggregate mineralogies and reactivities, including assessment of more aggregates when assessment by AMBT and CPT is ambiguous. In this research, the nonlinearity measurements for an aggregate that was near the expansion limit and was determined to be potentially deleterious and reactive by AMBT and CPT, respectively, showed negligible nonlinearity. The lack of ASR damage was confirmed through petrographic assessment. Assessment of more aggregates that are known to be challenging to classify is warranted, in particular.

In addition, with further NIRAS assessment it is also important to continue to assess the relationship between microstructural changes and changes in nonlinearity, as well as to compare results with standard expansion tests and field performance. This correlation among the various measures aids not only in the classification of the aggregates, but also in the continued assessment of the proposed limit on nonlinearity. In addition, a limit on cumulative nonlinearity might also result from further research and development efforts. In particular, the age at which aggregate reactivity is identified by NIRAS and expansion testing should be compared over a broader range of aggregates to provide additional understanding of the potential benefits of NIRAS for early and reliable detection of reactive aggregates.

Based on those results, a critical next step would be to build a predictive model that captures the physics of the damage evolution of ASR. This physics- and chemistry-based material model could be used to interpret these experimental results and thus predict the resonance response of an ASR-damaged specimen or concrete. This work would result in refinements in the accuracy of the identification of reactive aggregate and form the foundation for a device for detection of ASR in the field.

A round-robin test series should also be initiated for concrete prism samples. Such a series would involve several laboratories using the NIRAS technique to measure nonlinearity and CPT expansion in a pre-determined set of aggregates; these might include some of those identified through continued research on aggregates whose reactivity is "ambiguous." Results could be used to establish precision and bias for the test and to refine a standard test procedure.

Because the NIRAS method appears to be insensitive to reactive aggregate size and because the technique was successfully applied to both ordinary concrete and concrete with SCMs, the method appears to have potential as a technique for screening job mixes. A research program should be established to measure change in nonlinearity over a broad range of concrete, varying the aggregate reactivity and gradation, water-to-cement ratio, and binder composition. Exposure could be done using CPT or other accelerated testing appropriate for concrete.